Living the Scientific Life (Scientist, Interrupted)

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[Mystery bird] Black Tern, Chlidonias niger surinamensis, photographed at Smith Point Hawk Watch, Texas. [I will identify this bird for you tomorrow]

Image: Joseph Kennedy, 15 August 2008 [larger view].

Nikon D200, Kowa 883 telescope TSN-PZ camera eyepiece 1/250s f/8.0 at 1000.0mm iso400.


Rick Wright, Managing Director of WINGS Birding Tours Worldwide, writes:

After all of those big, mean-looking terns, it’s nice to see one as sweet and gentle as this little bird. How do we know it’s little? The pebbles and grass provide a welcome sense of scale, but even without them, the large eye, small bill, and generally “soft” aspect eliminate the larger species of tern.

The large terns, too, with the obvious exceptions of the long-billed noddies, Sooty, and Bridled, tend to appear white in the field, while our quiz bird gives a neat two-toned impression: dark gray above, white below and on the head, with some odd dark spots on the head. The tail falls far short of the folded wingtip on the perched bird. Small, dark-mantled, patchy-headed, short-tailed: this has to be a marsh tern, of the genus Chlidonias.

The marsh terns — Black, White-winged, and Whiskered — are obviously different from other terns. They feed like giant swallows, swooping low over the water to pick up insects and small fish, rarely or never diving from a hovering headstart like the large, “white” terns. Distinctive as they are as a group, they can be challenging to identify to species, especially in plumages other than the distinctive breeding feather of adults. A complication little regarded by many birders is the fact that American Black Terns (surinamensis) are different in appearance from Old World Blacks (niger), more closely resembling White-winged Tern in some ways.

The upperparts of our quiz bird are too dark for Whiskered Tern, which is larger, bulkier, and heavier-billed in any event. Our bird’s black auricular spot is relatively isolated on a very white head, connected (one assumes) to the invisible spot on the other “cheek” by a narrow, diffuse headband — a general pattern that is classic for White-winged Tern. Once I’ve got that idea in my brain, I start to see a short bill (Black Tern is longer-billed), and I can almost convince myself that there is a reddish tinge to the tarsus, which now strikes me as long. The flank and breast are white, without the gray wash conspicuous on some Black Terns, and I think that the pale rump contrasts with the slightly darker tail.

On the other hand, the upperparts are quite slaty, perhaps too dark for White-winged Tern, and the “corner” of the folded wing and the side of the neck both show dark, creating the “double patch” typical of Black Tern. And we need to remember that the head pattern of surinamensis Black Tern can approach that of White-winged.

If I saw this bird in the field, I’d look at it very closely indeed. Though most records are from the east coast, White-winged Tern is a good candidate for vagrancy anywhere in North America , especially in late summer. My suspicion is that the upperparts color and “double patch” make this, as expected, a Black Tern; but me, I’d go home with some question marks in my notebook — and hope to end the day perhaps a little more knowledgeable about terns, even unidentified terns, than I’d started it.

Review all mystery birds to date.

Comments

  1. #1 Christopher
    September 26, 2008

    Black Tern?

  2. #2 bobk
    September 26, 2008

    Bridled Tern.

  3. #3 Jim
    September 26, 2008

    Small tern with an all black bill and dark crown coming down into a dark ear patch, combined with the dark mark on the side of the breast says Black Tern. Does dark carpal bar mean its an immature? First fall bird?

  4. #4 The Ridger
    September 26, 2008

    Wow. His wings look very uneven in that photo.

  5. #5 Joel
    September 27, 2008

    Bridled Tern

  6. #6 Smilodon
    September 27, 2008

    It’s a Black Tern, as outlined above. Bridled Tern never has the largely white head that this bird displays.

  7. #7 bobk
    September 27, 2008

    Bridled Tern.

    The following is based on Sibley’s.
    I realize this looks like a Black tern due to the posture of the neck (held up), general size impression (smaller than 16″ anyway), the black eye patch and white eye ring. However, I believe this is a Bridled due to the lack of black on the back of the neck (hence “bridled”), the black wing primaries and perhaps most convincing, the heavily notched tail which can be seen as a reflection in the water.
    I questioned my first Black impression because postures can change; size is a very difficult characteristic to determine from a photograph and the shape of the black on the head may be very variable. But I will admit the eye ring is hard to explain away.
    I settled on Bridled because of the notched tail – how can that be explained away?

  8. #8 bobk
    September 27, 2008

    One other thing I found convincing was the lack of grey on the chest. Sibley’s drawings of shows the 1st summer Bridled with a completely white chest but the juvenile and adult non-breeding Black is shown with some gray extending around the wing to the chest.
    Please let us know if your experience contradics what Sibely has drawn. Drawings are never perfect.

  9. #9 bobk
    September 27, 2008

    OK, I must admit the jizz here say’s Black.

  10. #10 bobk
    September 27, 2008

    Where’s Rick?

  11. #11 "GrrlScientist"
    September 27, 2008

    sorry, i was a little late in publishing rick’s analysis. i’ve had some trouble finding a stable wifi connection.

  12. #12 tween
    September 27, 2008

    black tern

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